This invention relates to a method for anodically bonding a multilayer device with a free proof mass or other mass.
MEMS devices with free proof masses such as accelerometers and gyroscopes typically mount the free proof mass structure to the base layer using anodic bonding. The base layer typically includes a sense plate or electrode to sense the movement of the proof mass. Anodic bonding applies a high voltage e.g. 1000 volts across the free proof mass structure and base layer to effect the bond. In order to prevent the high voltage from attracting and bonding the free proof mass to the base layer, the free proof mass is kept in its bulk form until after the anodic bonding: the strength of the unfinished free proof mass structure is sufficient to prevent it from being flexed or drawn into contact with the base layer. Then, after bonding, the bulk of the free proof mass structure is removed leaving the suspended free proof mass. In order to improve the signal sensed from the motion of the proof mass a second, top, layer with another sense plate or electrode is mounted on the other side of the free proof mass opposite the base layer. Now any motion of the proof mass is sensed by both sense plates effectively doubling the signal strength. However, anodic bonding of this second layer presents a problem because now the proof mass is indeed free and applying the anodic bonding voltage across the layers will cause the free proof mass to be attracted and bonded to one of the layers. To combat this problem it has been suggested to ground the sense plates and the proof mass to prevent the attraction and bonding of the proof mass during anodic bonding. N. Ito, K. Yamada, H. Okada, M. Nishimura, T. Kuriyama, A Rapid and Selective Anodic Bonding Method, International Conference on Solid-State Sensors and Actuators, And Eurosensors IX, Proceedings V1, pg. 227-280, 1995. However, this solution introduces a substantial increase in complexity in the manufacturing process. Both the base and top sense plates must be connected to ground potential. In order to do this holes or vias must be made in the base and top layers, typically glass, which support the sense plates. These vias must be filled with metal or some conductor in order to establish an electrical connection between the sense plates and ground. In addition the proof mass, usually made of silicon must also be electrically connected to ground. All three of these electrical connections are difficult to effect and require several processing steps which add cost to the device. All of these vias and connections must be provided for each chip on a wafer which contains hundreds or even thousands of said chips. Further, after fabrication these ground connections must be removed to ensure reliable operation.
It is therefore an object of this invention to provide an improved method of anodically bonding a multilayer device with a free mass.
It is a further object of this invention to provide such an improved method of anodically bonding a multilayer device with a free mass which requires no additional, external vias or connections.
It is a further object of this invention to provide such an improved method of anodically bonding a multilayer device with a free mass which dramatically reduces complexity and increases yield.
It is a further object of this invention to provide such an improved method of anodically bonding a multilayer device with a free mass which requires no extra step in processing to undo connections.
This invention results from the realization that the free mass can be prevented from unwanted attachment during anodic bonding not just by grounding the effected parts which requires complex and potentially problematic connections and added processing steps but by simply electrically connecting the effected parts together internally to a floating potential which is easily cut when the wafer is diced into chips and more particularly that a multilayer device with a free mass can be more easily and simply anodically bonded by positioning a support layer on either side of the free mass with an electrode on each layer proximate the free mass, connecting both electrodes and the free mass, to a node at a floating potential and applying a voltage across the layers and more to bond at least one of the layers to the silicon layer.
This invention features a method of anodically bonding a multilayer device with a free mass including positioning a support layer on either side of a free mass structure which includes a free mass. There is an electrode on each layer proximate the free mass. Both electrodes and the free mass are connected to a node at a floating potential. A voltage is applied across the layers and the free mass structure to bond at least one of the layers to the free mass structure.
In a preferred embodiment the free mass may be a proof mass. The layers and free mass may include different materials. The layers may include glass and the free mass may include silicon. The node may be contiguous with the multilayer device. The multilayer device may be one of a plurality formed in a wafer structure. Each multilayer device may have a node associated with it. The wafer structure may be diced into individual multilayer devices and the dicing may disconnect the layers and free mass from the floating potential node. The wafer structure may include two wafers which form the support layers with a third wafer between them forming the free mass structure.